1) Field of the Invention
The invention relates to a copper alloy which has a low electric resistance, and can easily form a stable oxide film layer on its surface and its interface, and a liquid crystal display device using the copper alloy in wirings.
2) Description of the Related Art
While many metals suffer oxidation on their surfaces and hence deteriorate in performance, aluminum (Al) and alloys thereof form an Al oxide on their surfaces, and the Al oxide serves as a protective film which can prevent oxidation on the surface. With respect to iron (Fe), Fe itself does not form a protective film, but, in the form of stainless steel which is an Fe alloy containing chromium (Cr), Cr in stainless steel forms a protective film, which can prevent oxidation on the surface.
Cu is similar to Fe, and a Cu oxide is permeable to oxygen, and hence has no ability to prevent oxidation and cannot serve as a protective film. When an appropriate element is added to Cu to obtain a Cu alloy, like Cr in stainless steel, the alloying element in the Cu alloy possibly forms a protective film, which prevents Cu from suffering oxidation. Further, when the protective film is formed at the interface between the film and the adjacent solid, inter-diffusion is possibly prevented.
With respect to the studies on formation of a protective film which prevents oxidation of Cu, many reports have been made from the viewpoint of application to wiring materials for use in electronic devices, which are required to have high electrical conductivity.
For example, Japanese Patent Application Laid-open No. H05-047760 (Patent document 1) entitled “Semiconductor integrated circuit device having Cu wiring on semiconductor substrate” discloses “the Cu wiring is comprised of a Cu alloy containing an element having an electronegativity equal to or larger than that of Cu in an amount in the range from 0.01% by weight to less than 10% by weight”.
Japanese Patent Application Laid-open No. H11-054458 (Patent document 2) entitled “Metallization structure” discloses “a first step of depositing by sputtering a copper alloy, which comprises copper and an alloying element in an amount of less than 10 at. %, on a dielectric portion of a substrate having an oxide, and a second step of depositing a layer of copper on the copper alloy”.
Japanese Patent No. 2862727 (Patent document 3) entitled “Sputtering target for forming metal thin film and method for producing the same” discloses “a target material made of a high-purity copper alloy comprises a base metal of high-purity copper having a purity equal to or more than 99.9999% by weight, and 0.04 to 0.15% by weight of titanium having a purity equal to or more than 99.9% by weight added to the base metal”.
Japanese Patent No. 3220760 (Patent document 4) entitled “Semiconductor device” discloses a semiconductor device which is formed with “a copper compound having such a high corrosion resistance that the relative oxidation number is equal to or less than 50% of that of copper”.
Furthermore, “Thin Solid Films”, by W. A. Lanford et al., vol. 262 (1995), pp. 234-241 (Nonpatent literature 1) discloses “A copper alloy comprising copper and aluminum or magnesium, which prevents oxidation of copper”.
However, the electronegativity described in the Patent document 1 is not sufficient to form an oxide film having high adhesion to the Cu surface. The specification of the Patent document 2 describes that a Cu-2 at. % Mg alloy is heated in air at 400° C. for 30 minutes, so that the oxidation rate of Cu is considerably lowered. When a material having a Cu-(0.05 to 6.0) at. % Mg alloy thin film formed on silicon oxide (SiO2) is subjected to heat treatment at a temperature equal to or less than 400° C., Mg diffuses to the interface between the copper alloy and SiO2 during the heat treatment to form a protective film, thus preventing inter-diffusion between Cu and the SiO2 constituent elements. However, it has been known that a Cu—Mg alloy being heated at 600° C. reduces SiO2 to diffuse Si atoms into Cu, increasing the electric resistance of Cu. In addition to the reports on the copper alloy containing Mg, there are reports on the formation of protective films from alloys containing Al or Ti, but these alloying elements have a drawback in that they increase the electric resistance of Cu. The Patent document 3 has a problem in that titanium remains as a solid solution element or forms an inter-metallic compound to increase the electric resistance. In the Patent document 4, a copper compound having such a high corrosion resistance that the relative oxidation number is equal to or less than 50% of that of copper is formed on the surface of copper, which poses a problem in that the productivity is low due to the formation of a compound. The Nonpatent literature 1 has a problem in that both the Cu—Mg alloy and Cu—Al alloy subjected to heat treatment are increased in electric resistance.
Recently, in substrates having mounted electronic parts, such as a resistance, a capacitor, and a quartz clock, semiconductor elements, such as a TFT, a CMOS, and an FRAM, and semiconductor devices having mounted them, examples of wiring materials conventionally used include high melting-point metals, such as Mo and Cr, but these metals have a problem in that they have low conductivity. When, instead of them, aluminum (Al) having high conductivity is used in wirings, the conductivity can be considerably increased. In addition, aluminum (Al) and alloys thereof form Al oxide on their surfaces, which serves as a protective film to prevent oxidation on the surface. However, Al poses problems of the formation of Hillocks and the occurrence of electromigration.
For solving the problems, for example, Japanese Patent Application Laid-open No. 2000-199054 has proposed an Al—Nd alloy, which is used in advanced display devices. Al—Nd has an electric resistance lower than that of Cr; however, Al and Nd do not form solid solution, making it difficult to produce a sputter target. Further, the Al—Nd alloy contains Nd, which disadvantageously makes it difficult to lower the electric resistance of the alloy.
On the other hand, materials having higher conductivity are demanded, and, for meeting the demands, wirings comprised of a Cu alloy are being studied. Particularly, Cu has an electric resistance lower than that of Al and is unlikely to form Hillocks, and therefore Cu is a promising wiring material in the next generation. However, Cu has a problem in that it has poor adhesion to a substrate of glass, for example, and is likely to suffer oxidation. In addition, Cu has another problem in that Cu and an insulating layer of SiO2 or SiN undergo inter-diffusion and thus the electric properties deteriorate. For solving these problems of Cu wiring, Japanese Patent Application Laid-open No. 2004-139057 has proposed a method for forming around Cu a nitride having a high melting point, such as TaN, TiN, or WN. Differing from conventional wiring materials, however, this method requires a material for barrier layer and an additional process for forming the barrier layer and poses a problem in that the barrier layer having a high resistance and a large thickness is formed to increase the effective resistance of the wiring. Japanese Patent Application Laid-open No. 2004-91907 has proposed that 0.1 to 3.0 wt. % of Mo is added to Cu so that Mo segregates in the grain boundary, preventing oxidation due to the grain boundary diffusion. This method improves Cu in oxidation resistance, but involves a problem that the wiring resistance is increased.
Especially in the semiconductor devices, in recent years, liquid crystal display devices are widely used in a variety of fields of displays wherein the liquid crystal display devices have advantages in that they are small in thickness and lightweight and can be driven with a low voltage and cause small power consumption. The liquid crystal display device generally has a structure such that liquid crystal is sealed between two transparent glass substrates. A black matrix, a filter, a common electrode, and an oriented film are formed on the inner surface of one substrate, and a thin film transistor (TFT), a gate wiring, a signal wiring, a pixel electrode, and an oriented film are formed on the inner surface of another substrate. For example, three pixel electrodes are positioned to color filters of the three primary colors to define one pixel unit, and a number of the pixel units are arranged in rows and columns to constitute a color display in a plane form. An actual image is formed by a procedure in which the rows of pixels are successively selected by the gate wiring and an image signal is fed from the signal wiring to the pixel electrodes on the same row at the same time to operate the three primary-color pixel electrode of each pixel, defining an image. The TFT used in the liquid crystal display device is produced as follows. A gate wiring is first formed on a substrate and covered with a gate insulator film, and then an amorphous silicon layer constituting a channel layer is formed. A channel protecting film is formed in the channel region of the amorphous silicon layer, and then an amorphous silicon layer for contact having a high impurity density, and metal layers constituting a source/drain electrode and a signal wiring are formed in source/drain regions on the both sides of the channel region and patterned, and then covered with an insulating protective film. The operation speed of the TFT in the liquid crystal display device largely depends on the conductivity of a gate wiring and the capacity generated in the gate wiring. Increasing the conductivity of the gate wiring can improve the operation speed.
With respect to the display device, Japanese Patent Application Laid-open No. 2003-277852 discloses a copper metallizing composition comprising, relative to 100 parts by mass of copper powder, 0.5 to 8 parts by mass of a glass component and 0.05 to 3 parts by mass of a composite oxide comprised mainly of Zn, Mg, and Ti. There can be provided a copper metallizing composition which is advantageous in that it has improved adhesion strength and excellent solder wettability and suppresses warpage of a wiring substrate, and a ceramic wiring substrate using the composition.
Japanese Patent Application Laid-open Nos. 2003-332262 and 2003-342653 disclose a wiring material comprised of a Cu alloy for use in a wiring substrate, wherein the Cu alloy comprises any one of Au and Co or both and Cu, wherein the Cu content is 80 to 99.5 wt % and the sum of the Au content and the Co content is 0.5 to 20 wt %. There can be provided an alloy having improved adhesion to a glass substrate or silicon film. Japanese Patent Application Laid-open No. H10-153788 discloses a liquid crystal display device which comprises a wiring layer having a conductive layer comprising a material comprised mainly of at least one first metal selected from the group consisting of Ag, Au, Cu, Al, and Pt and comprised of at least one second metal selected from the group consisting of Ti, Zr, Hf, Ta, Nb, Si, B, La, Nd, Sm, Eu, Gd, Dy, Y, Yb, Ce, Mg, Th, and Cr, and an oxide layer covering the surface of the conductive layer and comprising a material comprised mainly of the second metal. High resistance to chemical treatment in the production process and excellent adhesion to the substrate can be achieved, thus preventing the wiring layer from suffering burnout.
However, in the above disclosed techniques, Cu oxide is permeable to oxygen and hence has no ability to prevent oxidation and cannot serve as a protective film. Especially when an appropriate element is added to Cu to obtain a Cu alloy, an additive element possibly forms a protective film which can prevent oxidation of Cu. However, the additive element does not diffuse satisfactorily and is difficult to form a strong oxide film.
The disclosed techniques have problems in that, when Cu is in contact with a gas atmosphere containing oxygen or a solid material containing oxygen, oxidation of Cu or inter-diffusion between Cu and the adjacent solid phase inevitably occurs, so that the high conductivity inherent in Cu cannot be maintained. For solving these problems, an attempt is made to add an alloying element to Cu to form a protective film, but other problems occur in that the resultant Cu alloy has poor thermal stability and an increased electric resistance.
The invention has been achieved in view of the above, and an object is to provide a highly conductive Cu alloy which is advantageous in that an alloying element added to Cu is first reacted with oxygen contained in a gas atmosphere or solid in contact with the Cu member to form an oxide film which can prevent oxidation of Cu.
The conventional wiring materials have a problem in that they cannot produce a liquid crystal display device which comprises a wiring layer having an oxide layer having high conductivity and having a strong protective layer on the surface of a wiring.
The invention has been achieved in view of the above, and another object of the invention is to provide a semiconductor device, particularly a liquid crystal display device comprising a highly conductive wiring or electrode which is advantageous in that the alloying element added to Cu is first reacted with oxygen contained in a gas atmosphere or solid in contact with the Cu member to form an oxide film which can prevent oxidation of Cu.